1. Field of the Invention
The present invention relates to a method of repairing superalloy components. More specifically, the invention provides a method of local heat treatment of superalloy components prior to welding in a manner that resists recrystallization of the material in locations where repair is not necessary, and also resists cracking in the heat affected zone of the weld and deposited weld metal while preserving the material properties of the remainder of the component.
2. Description of the Related Art
Components of various types of equipment that are subject to high temperature, high stress environments, for example, components within combustion turbines, are typically made from materials known as superalloys, which are defined herein as nickel based alloys containing aluminum and/or titanium, or cobalt based alloys. Components made from these materials typically include equiaxed materials, directionally solidified materials, or single crystal materials. After casting, the components are typically subjected to various heat treatments, for example, homogenization, hot isostatic pressing, solutionizing, and/or aging. The heating rate, hold temperature, hold time, and cooling rate of these heat treatment processes are intended to produce optimally sized and shaped grains of precipitate of Ni3(Al,Ti) and carbides within the material. The volume percentage, size, and distribution of these precipitates, along with the type and distribution of the carbide, determine the mechanical properties of the material. An optimum volume percentage and distribution of precipitates is the source of the material's high temperature strength.
During the operation of a combustion turbine having such components therein, the high temperature and stress to which the components are subjected cause precipitation of carbides in the grain boundaries in equiaxed and directionally solidified materials, and also causes coarsening of the Ni3(Al,ti) precipitates, thereby changing the mechanical properties of the material. Prolonged exposure to such conditions may cause cracking within the material.
Such cracks are typically repaired by welding, however, superalloys are difficult to weld. During welding, hot cracking may occur in the heat affected zone due to liquation of low melting phases such as borides, carbides, sulfides and/or phosphides in the grain boundaries. Present efforts to reduce hot cracking include design of the weldments, controlling trace elements within the base metal, using lower strength weld filler metals, and using welding processes with low heat inputs.
Additionally, post weld heat treatment cracking, also known as “strain age cracking,” may occur during the post weld heat treatment which is performed to restore the properties of the components and to relieve residual stresses within the material. Such cracks may extend beyond the heat affected zone through the weld metal or through the parent material. During heat treatment, as residual stress is relaxed, precipitation of Ni3(Al,Ti) occurs rapidly, resulting in volume contraction and strengthening of the material, thereby resulting in a reduction of the ductility of the material. Cracking occurs when the strain associated with stress relaxation exceeds the strain capacity of the heat affected zone. Hot cracks may act as the initiation points for strain age cracks.
The strain-age cracking tendency of superalloys is related to the total amount of alloying elements such as Al and Ti contained within the alloy.
Presently used methods to minimize strain age cracking include solution and overaging pre weld heat treatments. The former method works well with alloys with low Ni3(Al,Ti) volume percents, while the latter method works best for materials with high Ni3(Al,Ti) volume percent. Such heat treatment typically involves heating the entire component in a vacuum furnace to a predetermined temperature and cooling the component to room temperature, with the cooling done quickly or slowly depending on the desired result. A typical hold temperature is the solution temperature where all the Ni3(Al,Ti) precipitates go into solution.
In the case of directionally solidified or single crystal materials, the heat treatment hold temperature is limited to temperatures that are lower than the solution temperature due to recrystallization (formation of new small grains) within the material. Formation of recrystallized grains results in a reduction of the desired mechanical properties of the material. However, such low temperature heat treatment is insufficient to improve the weldability of the material.
Accordingly, there is a need for a method of heat treating superalloys in a manner that improves the weldability of the portion of the component to be repaired without damaging the microstructure and material properties of the remainder of the superalloy component. Such a method would substantially reduce the cost of maintaining equipment using superalloy components by improving the repairability of these components, and avoiding the expense of replacement of damaged components.